There are two common systems of rice cultivation in use today: the dry system wherein the crop is grown on dry ground and the wet system wherein the crop is planted, or transplanted, and grown in a flooded field. The wet system is the most prevalent due to the severe weather patterns in the major rice growing areas of the world. For example, over 90% of the world rice production is grown in Asia which has a pronounced monsoon season.
Although flooding itself can help reduce weed populations, some fields, such as those in large areas of south and southeast Asia, cannot be flooded continuously and therefore effective weed control practices are essential for efficient farming. The usual procedures for herbicide application involve removing the water from the field one or two days before the application of the herbicide; applying the herbicide; and then flooding the field again approximately two days after the herbicide has been applied.
Flooding can significantly affect the distribution, stability and residual activity of an herbicide in the soil. The anaerobic conditions common in flooded soils can adversely affect the chemical stability, and consequently the utility, of the herbicide. Furthermore, an herbicide will not necessarily remain in the soil layers of a flooded field. Movement of the herbicide from the soil into the water and the potential loss of the herbicide by codistillation can severely reduce the herbicide's effectiveness. The discovery and development of herbicides that are useful in this unique situation are therefore important concerns.
Certain substituted cycloalkenyl acetamides are useful as active ingredients in herbicidal compositions. Such herbicidal acetamides are disclosed, for example, in U.S. Pat. No. 3,586,496; 4,319,918 and 4,351,667 and in German Patent Application No. DE 3245020A1. Prior to the present invention, however, none of these known herbicidal cycloalkenyl acetamides exhibited an acceptable level of selectivity with respect to crops of flooded rice.
Accordingly, it is an object of the present invention to provide novel herbicidal cycloalkenyl acetamides which combine broad spectrum activity, high potency and improved selectivity for crops of cultivated plants. Moreover, it is an object of this invention to provide a novel class of herbicides which exhibit a remarkable degree of selectivity with respect to crops of flooded rice as well as dry-land crops.